This invention relates to a cementless prosthetic bearing element. More particularly it relates to a cementless prosthetic acetabular cup.
At the present time wear of the well known polyethylene acetabular bearing components in hips, the tibial bearing component in knees, and other prosthetic bearing inserts limits the life of these artificial joints. Normally orthopedic devices for joint construction and reconstruction comprise polyethylene bearings on metal or ceramic, or metal on metal for articulation. The polyethylene bearing is often held in a metal shell, for example in hip constructions or a tray in knee constructions. Conveniently bearing liners made from ultra-high molecular weight polyethylene are fixed via a snap fit into the metal backing.
The possible use of elastomers in bearings to encourage fluid film lubrication at low velocity, and with a low viscosity lubricant such as in a reconstructed joint has been published in laboratory studies which show that such xe2x80x9ccompliantxe2x80x9d bearings provide lower coefficient of friction compared with standard polyethylene versus metal bearings. Soft grades of polyurethane have been shown to be suitable materials for compliant bearings in, for example, EP 0 608382 and U.S. Pat. Nos. 5,879,387 and 6,221,108.
Because of the fluid or thin film lubrication, it is far less likely that the two bearing surfaces touch during use and thus wear is lower. It is however difficult to apply this concept for a number of reasons. One difficulty lies in selecting a polyurethane or other elastomer which will not be degraded by the human body so great care must be taken in selecting compliant bearing material. As mentioned above polyethylene bearing are usually held by physical means in, for example, a metal shell but it is not possible to use this approach with a xe2x80x9ccompliantxe2x80x9d bearing because as its stiffness is so much lower than the metal it would be possible for it to extrude from the shell or be easily moved relative to the shell causing damage. Also lower stiffness would lead to greater interface strain.
Because of the above difficulties, present inventors have tried several different approaches. Initially, for ease of use and conventional appearance, it was attempted to bond a soft grade of polyurethane onto a stiffer polyurethane backing to form a construct similar to pre-existing polyethylene components. This construct could then be assembled in a metal shell or cemented depending on the surgeon""s preference and clinical judgment.
The present invention therefore is related to a prosthetic bearing element which uses an elastomeric xe2x80x9ccompliantxe2x80x9d bearing material.
U.S. Pat. No. 5,879,387 describes a bearing element which has a backing made from a rigid polymeric material and a soft elastomeric polyurethane material liner. Bearing elements of this type can be made by a molding process and this is also described.
Similarly, U.S. Pat. No. 5,609,646 describes a process for the manufacture of an acetabular cup of particular configuration and in which the inner bearing component and the backing are molded together to form a single component, the backing being stiffer than the inner bearing component.
It has been found that the use of a molding technique to make such prosthetic bearing elements can be used to advantage to produce elements which have superior qualities and which do not require machining on the external surface of the backing.
It is known, for example from EP 0 297 789 to provide bearing elements of this type which are to be held in place by cement with concentric circumferential and radial grooves to assist in stabilizing and securing the shell stiffener to the layer of cement when the cup is placed in the acetabular socket. It is relatively easy to machine such grooves.
The present invention is intended to provide a cementless prosthetic bearing element which has been molded and which is possible to place in an acetabular socket without cement, the outer surface of the backing being being provided with elements to assist stabilization and the ingrowth of bone. Thus, the bearing element according to the present invention is cheaper and easier to produce and provides superior location and installation in the bone.
According to the present invention a cementless prosthetic bearing element comprises a backing which supports a bearing liner having a bearing surface, the backing being molded from a rigid polymeric material. The bearing liner is made from elastomeric polyurethane material and to which the backing is molded. The backing has an outer surface which includes a number of raised engagement features provided in directions extending away from and/or towards the outer periphery of the outer surface.
In one preferred embodiment the engagement features are formed by projecting strakes and these can be of substantially triangular cross-section.
The invention can be applied to bearing elements, the outer surface of the backing of which can be of any desired shape, for engagement with the bone concerned and if the element is in the form of an acetabular cup or similar device the outer surface of the backing can be substantially part-spherical and the raised engagement features extend in radial and/or chordal directions.
In order to provide additional stability the outer surface can also carry one or more projecting fins which extend normal to the surface. Such fins can be spaced apart and extend in parallel chordal directions.
It will be appreciated that the time and expense involved in attempting to machine such a surface having the kind of features referred to and such fins is extremely costly and time consuming whereas by applying the molding technique of the present invention they can be formed relatively easily.
The teaching set forth in U.S. Pat. No. 5,879,387, which are incorporated herein by reference, can be applied to the present invention and thus the backing can be made from a xe2x80x9crigidxe2x80x9d polymeric material having a minimum hardness value of 65 N/mm2 and the bearing liner can be made from a xe2x80x9csoftxe2x80x9d elastomeric polyurethane having a hardness value of 3.0 to 9.0 N/mm2 using hardness testing method BS 2782; PT3 Method 365D.
The bearing liner is preferably molded to the backing to form a single component and the backing can be made from polyurethane, for example Corothane 75D. Alternatively the backing can be made from carbon fiber reinforced plastics material, for example polybutyleneterphthlate (CFR-PBT), or an alloy of CFR-PBT with polyurethane. Again, the backing material can be made from polyetheretherketone (CFR-PEEK).
With yet another construction the backing material can be made from polymethylmethacrylate, all of these materials being suitable for molding. Preferably the bearing liner is made from Corothane 80A. Ideally the bearing liner will be bonded to the backing, but it may also be retained by a mechanical interlock between the two components.
As mentioned above, the element can be in the form of an acetabular cup. Thus, the embodiment shown in U.S. Pat. No. 5,609,646 the teachings of which are incorporated herein by reference, can be applied, that is, the backing may comprise a substantially part-spherical main portion and two independent arms projecting therefrom and formed by a separation or opening in the rim of the backing.
With this arrangement the inner bearing surface of the bearing liner is substantially part-spherical over a portion thereof spaced substantially opposite the separation or opening between the arms of the backing.
Preferably the bearing component has independent arms similar to the backing with a separation or opening between them and preferably the bearing component is of substantially the same configuration as the backing.
Alternatively the bearing component can be substantially hemispherical and extend across the separation or opening between the arms of the backing.